Some high intensity discharge lamps utilizing metal vapors as the discharge medium require a starting voltage much higher than the operating voltage and this is particularly so with high pressure sodium vapor lamps. For such lamps starting aids have become available which are combined with otherwise conventional ballasts to generate a series of high frequency pulses which initiate the breakdown in the lamp.
One very effective starting aid is described in U.S. Pat. No. 3,917,976--Nuckolls, Starting and Operating Circuit for Gaseous Discharge Lamps. It comprises a charging capacitor and a voltage sensitive switch device such as a sidac which are connected to form a series discharge loop with a number of turns at the output end of the ballast. This means that the output or secondary side of the ballast must have a tap giving access to some of the end turns and this is not always feasible or practical. In such case a discrete or remote type starter aid must be used.
In the case of series ballasts for street lighting connected in a current loop regulated by a moving core constant current transformer, heavy insulation is required between the windings and the core. This makes the coupling poor at high frequencies and the provision of a properly insulated tap difficult. Also the secondary of the series ballast does not have very many turns, and to use it as a pulse transformer may place excessive voltage stress on the insulation between turns. Hence it is generally desirable to use a discrete starting aid with series ballasts.
Discrete starters are also used in installations where the ballast is located at some distance from the lamp. Examples are freezer lockers where the ballast and its attendant heat losses are kept out of the cold compartment in which the lamp is installed, and outdoor lighting fixtures using slender poles where the ballast is located at the base of the pole in order to keep the weight down. Since the starting pulses have a fast rise time and contain high frequencies, they are rapidly attenuated in the transmission from ballast to lamp. Accordingly for such installations discrete starters are preferably utilized.
In a discrete starter, a pulse transformer is provided whose secondary is connected in series with the lamp across the ballast secondary terminals. The primary of the pulse transformer is connected in series with a voltage breakdown device across a capacitor, and circuit means are provided to charge the capacitor. When the device breaks down, the rush of current through the primary generates a high voltage low energy pulse in the secondary of the pulse transformer. The pulse is applied in series with the ballast secondary voltage across the lamp electrodes. After the lamp has started, the ballast secondary voltage is no longer high enough to cause breakdown in the device and the starting circuit becomes quiescent. Such circuits are well-known and have been described in U.S. Pat. No. 3,235,769--Wattenbach.